Photothermographic materials

ABSTRACT

A compound having a nucleus of the formula: ##STR1## The compounds are suitable for use as image stabilizers and antifoggants in photothermographic materials and exhibit acceptably low sensitization of human skin.

FIELD OF THE INVENTION

This invention relates to photothermographic materials and in particularto stabilisers and antifoggants for use therein.

BACKGROUND TO THE INVENTION

Heat-developable photosensitive materials which can produce photographicimages using a dry heat processing method (referred to herein as"photothermographic materials") are known and described for example, inU.S. Pat. Nos. 3,152,904 and 3,457,075 and in "Thermally ProcessedSilver Systems" by D. Morgan and B. Shely, "Imaging Processes andMaterials", Neblettes's Eighth edition; Edited by J. M. Sturge, V.Walworth and A. Shepp, p.2 (1969). Such photothermographic materialshave a photosensitive medium comprising a reducible silver source, e.g.an organic silver salt; a catalytic amount of photocatalyst, e.g.,silver halide, in reactive association with the reducible silver source,and a reducing agent for silver ion, ordinarily dispersed in an(organic) binder matrix. Although stable at ambient temperatures, whenheated to higher temperatures, e.g., 80° C., or higher, after imagewiseexposure, silver is produced in the exposed regions of the medium via aredox reaction between the reducible silver source and the reducingagent. This redox reaction is accelerated by the catalytic action of theexposure-generated silver catalyst. The silver provides a black imagewhich contrasts with the unexposed areas, resulting in the formation ofan image.

In practice, it is essential to include an effective antifoggant in suchphotothermographic materials, since without an antifoggant, somegeneration of silver in the unexposed areas takes place upon thermaldevelopment, resulting in a poor differential between image andbackground fog. In addition, one of the problems of photothermographicmaterials involves their post-processing stability. Since the process isperformed without a fixing step, it is desirable to provide a means toenable room light handling of the final image.

Polybrominated organic compounds have been described as bothantifoggants and image-stabilisers for photothermographic materialssince they can oxidise reduced silver (fog) back to silver bromide underthermal (anti-foggant) and light exposed (image stabiliser) conditions.

Examples of such compounds are described in U.S. Pat. No. 4,546,075,U.S. Pat. No. 4,452,885 (tribromomethylheterocycles as anti-foggants),U.S. Pat. No. 3,874,946 (tribromomethylsulphonylaromatics as imagestabilisers) and British Patent Application No. 9221383.4(tribromomethylketones).

Due to the thermal processing of the photothermographic imagingmaterial, all the compounds used in the construction of the materialwill be present in the final image sheet. All the materials included insuch a material must be acceptable both environmentally and in theirpossible effect on those persons who might come into contact with thematerial. The materials must be determined to be non-mutagenic and alsoit is highly desirable that they do not sensitise the skin of those whocome in contact with them. Many polybrominated organic compounds areknown to be powerful skin irritants and sensitisers and it is desirableto find compounds having effective anti-foggant and image stabilityproperties for photothermographic materials which exhibit acceptably lowsensitisation to human skin.

Our copending British Patent Application No. 9300147.7 disclosescompounds of the formula: ##STR2## in which;

R represents a hydrogen atom, an alkyl group, an aryl group or aheterocyclic group, any of which groups may be substituted.

This small class of compounds are effective antifoggants and imagestabilizers in photothermographic materials and exhibit low skinsensitisation. The latter property is particularly surprising sinceother compounds of similar structure have proved positive in skinsensitisation tests.

The present invention provides a further class of polybromo organicmolecules which are suitable for use as image stabilizers andantifoggants in photothermographic materials and exhibit acceptably lowsensitisation of human skin and guinea-pigs.

BRIEF SUMMARY OF THE INVENTION

According to the present invention there is provided a compound having anucleus of the formula: ##STR3##

The compounds of the invention generally have the formula: ##STR4##

The ring substituents R may be the same or different and selected fromany of those groups well known in organic chemistry.

DESCRIPTION OF PREFERRED EMBODIMENTS

Each R is generally selected from hydrogen, alkyl groups comprising upto 10 carbon atoms, preferably up to 5 carbon atoms; aryl groupscomprising up to 14 carbon atoms, preferably up to 10 carbon atoms; 5,6, 7 or 8-membered heterocyclic ring nuclei and heterocyclic fused ringnuclei comprising up to 14 ring atoms, halogen atoms (e.g., fluorine,chlorine, bromine and iodine), a hydroxy group, alkoxy groups (e.g.,methoxy, ethoxy etc.), aryloxy groups (e.g., phenoxy, hydroxyphenoxyetc.), amino groups (e.g., amino, methylamino, dimethylamino etc.), acyano group, acylamino groups (e.g., acetylamino, benzoylamino etc.),diacylamino groups (e.g., succinimido etc.), ureido groups (e.g.,methylureido etc.), sulphonamido groups (e.g., methylsulphonamido etc.),acyloxy groups (e.g., acetyloxy etc.), sulphamoyl groups (e.g.,N-ethylsulphamoyl etc.), alkylcarbonyl groups, arylcarbonyl groups,alkoxycarbonyl groups (e.g., methoxycarbonyl, ethoxycarbonyl etc.),aryloxycarbonyl groups (e.g., phenoxycarbonyl etc.), alkoxycarbonylamino groups (e.g., ethoxycarbonylamino etc.), hydroxyalkyl groups(e.g., hydroxyethyl, hydroxypropyl etc.), alkoxyalkyl groups (e.g.,methoxyethyl, methoxypropyl etc.), mercapto groups, alkylthio groups,arylthio groups, alkylsulphonyl groups, arylsulphonyl groups, acylgroups, aralkyl groups, alkyl groups containing a carboxyl group (e.g.,carboxymethyl, carboxyethyl etc.), each of which groups may whereappropriate comprise up to 14 carbon atoms, preferably not more than 10carbon atoms.

Examples of ring and fused ring nuclei represented by R include:isoxazole, pyrimidine, quinoxaline, indolenine and tetraazindene.

Examples of alkyl groups represented by R include: methyl, ethyl,propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, isopentyl, hexyl,octyl etc.

Examples of aryl groups represented by R include: phenyl, ethoxyphenyl,tolyl, xylyl, naphthyl etc.

As is well understood in this technical area, a large degree ofsubstitution is not only tolerated, but is often advisable. As a meansof simplifying the discussion, the terms "nucleus", "group" and "moiety"are used to differentiate between chemical species that allow forsubstitution or which may be substituted and those which do not or maynot be so substituted. For example, the phrase "alkyl group" is intendedto include not only pure hydrocarbon alkyl chains, such as methyl,ethyl, octyl, cyclohexyl, iso-octyl, t-butyl and the like, but alsoalkyl chains bearing conventional substituents known in the art, such ashydroxyl, alkoxy, phenyl, halogen (F, Cl, Br and I), cyano, nitro, aminoetc. The term "nucleus" is likewise considered to allow forsubstitution. Thus, the phrase "pyrimidine nucleus" would be understoodto include not only an unsubstituted pyrimidine ring, but alsopyrimidine rings bearing conventional substituents known in the art. Thephrase "alkyl moiety" on the other hand is limited to the inclusion ofonly pure hydrocarbon alkyl chains, such as methyl, ethyl, propyl,cyclohexyl, iso-octyl, t-butyl etc.

The invention also relates to photothermographic elements comprisinglight-sensitive silver halide in reactive association with a reduciblesilver compound, a reducing agent capable of reducing the silvercompound to metallic silver, and, as an antifoggant, a compound of theinvention as defined above.

Conventional silver halide photothermographic chemistry is used in thematerials of the invention. Such chemistry is described in, e.g., U.S.Pat. Nos. 3,457,075, 3,839,049, 3,985,565, 4,022,617 and 4,460,681. Anyof the various photothermographic media, such as full soaps, partialsoaps, and the like may be used in the practice of the presentinvention, including both black-and-white and color chemistries andeither in situ halidised (e.g., as disclosed in U.S. Pat. No. 3,457,075)or preformed silver halide sources (e.g., as disclosed in U.S. Pat. No.3,839,049) may be used.

Conventional photothermographic chemistry comprises a photosensitivesilver halide catalyst, a silver compound capable of being reduced toform a metallic silver image (e.g., silver salts, both organic andinorganic, and silver complexes, usually light-insensitive silvermaterials), a developing agent for silver ion (a mild reducing agent forsilver ion) and a binder.

The photothermographic chemistry may be black and white orcolour-forming. In the latter type of material, the reducing agentgenerates a colour on oxidation, either by becoming coloured itself, orby releasing a dye during the process of oxidation. Any leuco dyecapable of being oxidized by silver ion to form a visible dye is usefulin the practice of the present invention. Dye-forming developers such asthose disclosed in U.S. Pat. Nos. 3,445,234, 4,021,240, 4,022,617,4,368,247 and 4,460,681 are useful, and also those disclosed in JapanesePatent Publication No. 82-500352, and likewise dye-releasing developers,such as those disclosed in U.S. Pat. No. 4,981,775.

The compounds of the invention may be incorporated into thephotothermographic medium in the same manner as antifoggants of theprior art. The optimum concentration for individual compounds of theinvention may vary widely. In some cases, starting from the minimumamount required to suppress fog, increasing the amount of thetribromomethyl sulphone compound leads to a loss of image density, butin other cases it may produce an increase in image density beforelevelling out. In general, the compounds of the invention are utilisedin amounts of from about 1×10⁻³ to about 1×10⁻¹ moles per mole ofsilver, although amounts outside this range may also be useful.

In addition to the compounds of the invention, the photothermographicmedia of the invention may also contain, as a speed enhancing agent, aheterocyclic ring compound of the type disclosed in U.S. Pat. No.5,028,523 in which a nitrogen atom of the ring is electrically balancedby hydrobromic acid and which is associated with a pair of bromineatoms. The central nucleus of the nitrogen-containing heterocycliccompound may be generally represented by any of the following formulae:##STR5## in which;

Q represents the atoms (preferably selected from C, S, N, Se and O, morepreferably C, N and O) necessary to complete a 5-, 6-, or 7-memberedheterocyclic ring (monocyclic) or fused ring nucleus (polycyclic,especially bicyclic, with a fused-on benzene ring). The heterocyclicnucleus may possess one or more substituents selected from those definedfor groups represented by R. Exemplary and preferred heterocyclic ringgroups include pyridine, pyrrolidone and pyrrolidinone. Other usefulheterocyclic ring nuclei include pyrocyclic rings, e.g., pyrrolidines,phthalazinone, phthalazine etc.

Preferred heterocyclic nuclei for use in the practice of the presentinvention may be defined by the formulae: ##STR6## in which;

n is 0 (zero) or has integral values of from 1 to 4, and

each R⁵ represents a substituent selected from those defined for groupsrepresented by R, e.g., alkyl groups, alkoxy groups, aryl groups, nitro,cyano, and the like. Substituents on adjacent positions may form fusedring groups so that formula (i) above would in fact be inclusive offormulae (ii) and (iv).

These compounds are generally used in an amount of at least 0.005moles/mole of silver. Usually the range is from 0.005 to 1.0 moles ofthe compound per mole of silver and preferably between 0.01 and 0.3moles per mole of silver. The preferred level is currently about 0.01moles/mole silver.

The preferred nitrogen-containing heterocyclic compound is pyridiniumhydrobromide perbromide (PHP).

Photothermographic materials are usually constructed as one or twoimaging layers on a substrate. Single layer constructions must containthe reducible silver source, the silver halide and the developer, aswell as optional additional materials, such as toners, coating aids andother adjuvants. Two-layer constructions must contain the reduciblesilver source and silver halide in one layer (usually the layer adjacentthe substrate) and the other ingredients in the second layer or bothlayers.

The silver halide may be any photosensitive silver halide, such assilver chloride, silver bromide, silver iodide, silver chlorobromide,silver bromoiodide, silver chlorobromoiodide etc., and may be added tothe imaging layer in any fashion which places it in catalytic proximityto the reducible silver source. The silver halide generally comprisesfrom 0.75 to 15% by weight of the imaging layer, although larger amountsof up to about 25% by weight, are also useful. It is preferred to usefrom 1 to 10% by weight silver halide in the layer, more preferably from1.5 to 7%. The silver halide may be prepared in situ by conversion of aportion of silver soap by reaction with halide ions or it may bepreformed and added during soap generation, or a combination of bothmethods may be used. The latter is preferred.

The silver halide may be sensitised to visible or infrared light bymeans of the appropriate dyes, which may be added to the mixture ofsilver halide and reducible silver salt, or when preformed silver halideis employed, spectral sensitisation may be carried out prior to mixingwith the reducible silver salt, as described in U.S. Pat. No. 4,476,220.Spectral sensitising dyes for use in photothermographic media are wellknown in the art, and include cyanine dyes and merocyanine dyes asdisclosed, for example, in U.S. Pat. Nos. 3,761,279, 3,719,495,3,877,943 and 4,835,096, and European Patent No. 127,455. A preferredclass of infrared sensitising dyes is disclosed in our copending BritishPatent Application No. 9305324.7, filed Mar. 16th 1993.

The reducible silver source may comprise any material which contains areducible source of silver ions. Silver salts of organic andhetero-organic acids, particularly long chain fatty carboxylic acids(comprising from 10 to 30, preferably 15 to 25 carbon atoms), arepreferred. Complexes of organic or inorganic silver salts in which theligand has a gross stability constant for silver ion of between 4.0 and10.0 are also useful.

Examples of suitable silver salts are disclosed in Research DisclosureNos. 17029 and 29963 and include: salts of organic acids, e.g., gallicacid, oxalic acid behenic acid, stearic acid, palmitic acid, lauric acidand the like; silver carboxyalkylthiourea salts, e.g.,1-(3-carboxypropyl)thiourea, 1-(3-carboxypropyl)-3,3-dimethylthioureaand the like; complexes of silver with the polymeric reaction product ofan aldehyde with a hydroxy-substituted aromatic carboxylic acid, e.g.,aldehydes, such as formaldehyde, acetaldehyde and butyraldehyde, andhydroxy-substituted acids, such as salicylic acid, benzilic acid,3,5-dihydroxybenzilic acid and 5,5-thiodisalicylic acid, silver salts orcomplexes of thiones, e.g.,3-(2-carboxyethyl)-4-hydroxymethyl-4-thiazoline-2-thione and3-carboxymethyl-4-thiazoline-2-thione complexes or salts of silver withnitrogen acids selected from imidazole, pyrazole, urazole,1,2,4-triazole and 1H-tetrazole, 3-amino-5-benzylthio-1,2,4-triazole andbenzotriazole; silver salts of saccharin, 5-chlorosalicylaldoxime andthe like; and silver salts of mercaptides.

The preferred silver source is silver behenate.

The reducible silver source generally comprises from 5 to 70%,preferably from 7 to 45% by weight of the imaging layer. The use of asecond imaging layer in a two-layer construction does not affect thepercentage of the silver source.

The reducing agent for silver ion may be any material, although organicmaterials are preferred which will reduce silver ion to metallic silver.Conventional photographic developers such as phenidone, hydroquinonesand catechol are useful, but hindered phenol reducing agents arepreferred. The reducing agent generally comprises from 1 to 10% byweight of the imaging layer, but in a two-layer construction, if thereducing agent is in the layer separate from that containing thereducible silver source, slightly higher proportions, e.g., from 2 to15%, tend to be more desirable. Colour photothermographic materials,such as those disclosed in U.S. Pat. No. 4,460,681, are alsocontemplated in the practice of the present invention.

Examples of suitable reducing agents are disclosed in U.S. Pat. Nos.3,770,448, 3,773,512 and 3,593,863 and Research Disclosure Nos. 17029and 29963, and include aminohydroxycycloalkenone compounds, e.g.,2-hydroxypiperidino-2-cyclohexenone; esters of amino reductones asdeveloping agent precursors, e.g., piperidino hexose reductonemonoacetate; N-hydroxyurea derivatives, e.g.,N-p-methylphenyl-N-hydroxyurea; hydrazones of aldehydes and ketones,e.g., anthracene aldehyde phenylhydrazone; phosphoramidophenols;phosphoramidoanilines; polyhydroxybenzenes, e.g., hydroquinone,t-butyl-hydroquinone, isopropylhydroquinone and(2,5-dihydroxyphenyl)methylsulfone; sulfhydroxamic acids, e.g.,benzenesulfhydroxamic acid; sulfonamidoanilines, e.g.,4-(N-methanesulfonamido)aniline; 2-tetrazolylthiohydroquinones, e.g.,2-methyl-5-(1-phenyl-5-tetrazolylthio)hydroquinone;tetrahydroquinoxalones, e.g., 1,2,3,4,-tetrahydroquinoxaline;amidoxines; azines, e.g., a combination of aliphatic carboxylic acidaryl hydrazides and ascorbic acid; a combination of a polyhydroxybenzeneand a hydroxylamine, a reductone and/or a hydrazine; hydroxamic acids; acombination of azines and sulfonamidophenols; α-cyanophenylacetic acidderivatives; a combination of a bis-β-naphthol and a1,3-dihydroxybenzene derivative; 5-pyrazolones; sulfonamidophenolreducing agents; 2-phenylindane-1,3-dione and the like; chromans;1,4-dihydropyridines, such as2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridine; bisphenols, e.g.,bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, bis(6-hydroxy-m-toly)mesitol, 2,2-bis(4-hydroxy-3-methylphenyl)propane,4,4-ethylidene-bis(2-t-butyl-6-methylphenol, UV-sensitive ascorbic acidderivatives and 3-pyrazolidones.

The preferred developers are hindered phenols of the general formula:##STR7## in which;

R⁶ represents hydrogen or an alkyl group generally comprising up to 15carbon atoms, e.g., butyl, 2,4,4-trimethylpentyl etc, and

R⁷ and R⁸ represent H or alkyl groups of up to 5 carbon atoms, e.g.methyl, ethyl, t-butyl etc.

The presence of a toner (sometimes referred to as a "tone modifier") isnot essential, but is highly preferred. Examples of suitable toners aredisclosed in Research Disclosure No. 17029 and include: imides, e.g.,phthalimide; cyclic imides, pyrazolin-5-ones and a quinazolinone, suchas succinimide, 3-phenyl-2-pyrazolin-5-one, 1-phenylurazole, quinazolineand 2,4-thiazolidinedione; naphthalimides, e.g.,N-hydroxy-1,8-naphthalimide; cobalt complexes, e.g., cobaltic hexamminetrifluoroacetate, mercaptans, e.g., 3-mercapto-1,2,4-triazole;N-(aminomethyl)aryl dicarboximides, e.g.,N-(dimethylaminomethyl)phthalimide; a combination of blocked pyrazoles,isothiuronium derivatives and certain photobleach agents, e.g., acombination of N,N'-hexamethylene bis(1-carbamoyl-3,5-dimethylpyrazole),1,8-(3,6-dioxaoctane)bis(isothiuronium trifluoroacetate) and2-(tribromomethylsulfonyl) benzothiazole); merocyanine dyes, such as3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methylethylidene]-2-thio-2,4-oxazolidinedione;phthalazinone, phthalazinone derivatives or metal salts of thesederivatives, such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone,5,7-dimethoxyphthalazinone and 2,3-dihydro-1,4-phthalazinedione; acombination of phthalazinone and a sulfinic acid derivative, e.g.,6-chlorophthalazinone plus sodium benzene sulfinate or8-methylphthalazinone plus sodium p-tolysulfinate; a combination ofphthalazinone plus phthalic acid; a combination of phthalazine includingan adduct of phthalazine and maleic anhydride) and at least one compoundselected from phthalic acid, a 2,3-naphthalene dicarboxylic acid or ano-phenylene acid derivative and anhydrides thereof, e.g., phthalic acid,4-methylphthalic acid, 4-nitrophthalic acid and tetrachlorophthalicanhydride; quinazolinediones, benzoxazine and naphthoxazine derivatives;benzoxazine-2,4-diones, e.g., 1,3-benzoxazine-2,4-dione; pyrimidines andasym-triazines, e.g., 2,4-dihydroxypyrimidine, and tetraazapentalenederivatives, e.g.,3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetraazapentalene.

Preferred toners are phthalazinone, phthalazine, 4-methylphthalic acidand phthalic acid, either alone or in combination with other compounds.

The toner, when present, is generally included in an amount of from 0.2to 12%, preferably 0.2 to 5% by weight of the imaging layer.

The photothermographic chemistry is typically applied to the support ina binder. A wide range of binders may be employed in the imaginglayer(s), including both natural and synthetic resins. Copolymers andterpolymers are of course included. Suitable binders are transparent ortranslucent, are generally colourless and include natural polymers,synthetic resins, polymers and copolymers and other film forming mediasuch as: gelatin; gum arabic; poly(vinyl alcohol); cellulose esters,such as hydroxyethyl cellulose, cellulose acetate, cellulose acetatebutyrate; poly(vinyl pyrrolidone); casein; starch; poly(acrylic acid),poly(methylmethacrylic acid), poly(methacrylic acid); poly(vinylchloride); copoly(styrene-maleic anhydride),copoly(styrene-acrylonitrile), copoly(styrene-butadiene);polyacrylonitrile; polyvinyl acetals, such as, poly(vinyl formal) andpoly(vinyl butyral); polyesters; polyurethanes; phenoxy resins;poly(vinylidene chloride); polyepoxides; polycarbonates; poly(vinylacetate); polyolefins, such as poly(ethylene) and poly(propylene), andpolyamides. Poly(vinyl acetals), such as poly(vinyl butyral) andpoly(vinyl formal), and vinyl copolymers, such as poly(vinylacetate-chloride) are particularly desirable. The binders are generallyused in an amount ranging from 20 to 75% by weight, preferably from 30to 55% by weight of the silver halide containing layer. The binders maybe coated from aqueous or organic solvents or an emulsion.

The photothermographic elements of the invention are prepared by simplycoating a suitable support or substrate with the one or more imaginglayers containing the photothermographic chemistry and, optionally, aoxygen-barrier overlayer. Suitable barrier layers are well known in theart. Each layer is generally coated from a suitable solvent usingtechniques known in the art. Exemplary supports include materials, suchas paper, polyethylene-coated paper, polypropylene-coated paper,parchment, cloth and the like; sheets and foils of metals, such asaluminium, copper, magnesium and zinc; glass and glass coated withmetals such as chromium alloys, steel, silver, gold and platinum;synthetic polymeric materials, such as poly(alkyl methacrylates), e.g.,poly(methyl methacrylate), polyesters, e.g., poly(ethyleneterephthalate) and poly(ethylene naphthalate), poly(vinyl acetals),polyamides, e.g., nylon, cellulose esters, e.g., cellulose nitrate,cellulose acetate, cellulose acetate propionate, cellulose acetatebutyrate, and the like.

Various other adjuvants may be added to the photothermographic medium.For example, accelerators, acutance dyes, sensitizers, stabilizers,plasticizers, surfactants, lubricants, coating aids, antifoggants, leucodyes, chelating agents, binder crosslinking agents, UV-absorbers andvarious other well-known additives may be usefully incorporated in themedium. The use of acutance dyes matched to the spectral emission of theexposure source is particularly desirable.

It is not essential for the photothermographic elements of the inventionto comprise a separate support since each binder layer, together withthe photothermographic chemistry may be cast to form a self-supportingfilm.

The supports can be sub-coated with known subbing materials such as:copolymers and terpolymers of vinylidene chloride; and acrylic monomers,such as acrylonitrile and methyl acrylate; unsaturated dicarboxylicacids, such as itaconic or acrylic acid; carboxymethyl cellulose;polyacrylamide, and similar polymeric materials.

The support can also carry a filter or antihalation layer, such as onecomprising a dyed polymer layer, which absorbs the exposing radiationafter it passes through the radiation-sensitive layer and eliminatesunwanted reflection from the support.

The invention will now be illustrated by the following Examples.

EXAMPLE 1

Preparation of: ##STR8##

A solution of 2-quinolylthioacetic acid (6.97 g) and sodium hydrogencarbonate (2.73 g) in water (70 ml) was added slowly to a solution ofbromine (13.2 ml) and sodium hydroxide (20.85 g) in water (560 ml) whilemaintaining a temperature of less than 30° C. After four hours, aprecipitate formed that was collected by filtration and dried. Thissolid was recrystallised from ethanol to yield white needles (5.66 g41%) mp 171°-2° C.

EXAMPLE 2

Skin sensitivity testing

The compounds of this invention are covered by the general formula ofU.S. Pat. No. 3,874,946 which describes image stabilisers forphotothermographic materials but are not exemplified in that patent. Acompound that is exemplified in that patent is Compound B which we shalluse for comparison of skin sensitising properties with a compound ofthis invention (Compound A). ##STR9## Procedure for Repeat Insult HumanPatch Tests (RIHPT)

The purpose of this test is to evaluate potential for the induction ofallergic contact dermatitis. It is described in detail in: J Stotts,"Planning, conduct and interpretation of human predictive sensitisationpatch tests," in Current Concepts in Cutaneous Toxicology (V A Drill andP Lazer, eds), pg 41, Academic Press, New York 1980.

The RIHPT test determines the responses of human volunteers to inductionand challenge patch application of the compound of interest in a coated`Dry Silver` formulation on paper or PET film. The balance of theformulation has been shown in a separate RIHPT test not to cause contactdermatitis. Induction consists of nine repeated applications (threepatches, about one inch square, per week, for approximately 24 hours).Twelve to twenty four days after the final induction application,challenge patches are applied. Allergic contact dermatitis is evaluatedprimarily from the responses forty eight to ninety six hours afterchallenge application.

Compound A 0 of >200 persons showed contact dermatitis (not asensitiser)

Compound B 37 of 211 persons showed contact dermatitis; another 16 wereinconclusive.

(comparison) (extreme sensitiser)

Procedure for skin sensitivity tests (guinea-pig)

The guinea pig maximisation test protocol as described by S Magnussonand A Kligman in Allergic Contact Dermatitis in the Guinea Pig, C CThomas ed, pgs 11-117 (1970) was used. The compounds were screened forirritation. None of the compounds caused skin irritation to guinea pigsafter application as a 25% w/w suspension in petrolatum (mineral oil)for 24 hours.

A sensitisation test was conducted. For each compound, ten male guineapigs were assigned to the test group and four male guinea pigs wereassigned to the naive control group. On day 1, animals in the test groupreceived duplicate 0.05 ml intradermal injections of a 1:1 ratio ofFreund's Complete Adjuvant and sterile water on the shoulder area. Sixdays later, animals in the test group were pretreated with sodium laurylsulphate applied topically at the injection sites. On day eight, a 25%w/w mixture of the test compound in petrolatum was applied over theinjection sites of the animals and occluded for 48 hours. The naivecontrol animals were not treated during the induction phase.

Two weeks after the topical application, all animals received achallenge dose. A 25% w/w mixture of the test compound in petrolatum wasapplied to the right flank of test and naive control (previouslyuntreated) animals. All sites were occluded for 24 hours and then wipedclean. Test sites were examined for erythema reactions at 24 and 48hours after patch removal. In no case was reaction seen in the naivecontrol animals.

Compound A 0 out of 10 animals showed reactions

Compound B 10 out of 10 animals showed moderate to intense dermalreactions (extreme sensitiser)

EXAMPLE 3

A series of black and white photothermographic elements were prepared:Silver soap underlayer:

A silver halide-silver behenate dry soap was prepared by the proceduresdescribed in U.S. Pat. No. 3,839,049. The silver halide totalled 9% ofthe total silver while silver behenate comprised 91% of the totalsilver. The silver halide was a 0.055 micron silver bromoiodide emulsionwith 2% iodide.

A photothermographic emulsion was prepared by homogenising 300 g of theabove dry soap with 525 g of toluene, 1675 g 2-butanone and 50 gpoly(vinylbutyral) (B-76, Monsanto).

The homogenised photothermographic emulsion (500 g) and 100 g 2-butanonewere cooled to 55° C. with stirring. Additional poly(vinylbutyral) (75.7g B-76) was added and stirred for 20 minutes. Pyridinium hydrobromideperbromide (PHP, 0.45 g) was added and stirred for two hours. Theaddition of 3.25 ml of a calcium bromide solution (1 g of CaBr₂ and 10ml of methanol) was followed by 30 minutes of stirring. The temperaturewas raised to 70° F. and the following were added in 15 minuteincrements with stirring: IR dye solution (8.8 mg Dye 1 in 7.1 g DMF),4.2 g of supersensitiser solution (0.22 g 2-mercaptobenzimidazole and 4g methanol) and 16.6 g of developer1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane.

Topcoat: An active, protective top coat was prepared from the followingingredients;

    ______________________________________                                        acetone                  256 g                                                methylethylketone        123 g                                                methanol                 50 g                                                 cellulose acetate        20.2 g                                               phthalazine              2.89 g                                               4-methylphthalic acid    1.55 g                                               tetrachlorophthalic acid 1.01 g                                               tetrachlorophthalic anhydride                                                                          1.5 g                                                ______________________________________                                    

Compound A (this invention) or 0.125/0.188/0.25/0.312 g

Compound C (comparison) ##STR10##

The photothermographic emulsion was coated on 3 mil (7.6×10⁻⁵ m)polyester base by means of a knife coater and dried at 175° F. for fourminutes. The dry coating weight was 23 g per sq m.

The topcoat solutions were coated over the silver layer at a dry weightof 3.0 g per sq m. The layer was dried at 175° F. for four minutes.

Once dry the materials were imaged by exposure with a laser sensitometerincorporating a 780 nm diode. After exposure, the film strips wereprocessed at 260° F. for ten seconds. The images obtained were evaluatedby a densitometer. Sensitometric results include Dmin, Dmax, speed andcontrast. Sensitometry was also evaluated following accelerated aging at120° F and 50% relative humidity for 7 and 14 days. The results arereported in the following Tables.

    ______________________________________                                                    Dmin  Dmax     Speed   Contrast                                   ______________________________________                                        Compound A 0.12 g                                                                           0.12    3.07     2.72  3.52                                     Compound A 0.188 g                                                                          0.12    3.03     2.82  3.56                                     Compound A 0.250 g                                                                          0.11    2.97     2.80  3.56                                     Compound A 0.312 g                                                                          0.10    2.94     2.77  3.31                                     Compound C 0.250 g                                                                          0.10    2.95     2.72  3.52                                     (comparison)                                                                  No Compound   Black   Black                                                   ______________________________________                                    

After aging--120° F./50% RN

    ______________________________________                                                    7 Days          14 Days                                                       Dmin  Dmax      Dmin    Dmax                                      ______________________________________                                        Compound A 0.125 g                                                                          0.11    3.14      0.11  3.19                                    Compound A 0.188 g                                                                          0.11    3.10      0.11  3.19                                    Compound A 0.250 g                                                                          0.10    3.07      0.10  3.13                                    Compound A 0.312 g                                                                          0.10    3.05      0.09  3.06                                    Compound C 0.250 g                                                                          0.11    3.05      0.10  3.06                                    No Compound   Black             Black                                         ______________________________________                                         ##STR11##

Compound C is disclosed in our co-pending UK Patent Application No.9300147.7 and is shown therein to combine excellent photothermographicproperties (better than Compound B) with greatly reduced skinsensitising properties. The photographic properties of compound A of theinvention are shown herein to compare favourably with those of CompoundC.

We claim:
 1. A photothermographic element comprising a light-sensitivesilver halide in reactive association with a reducible silver compound,a reducing agent capable of reducing the silver compound to metallicsilver, and, as antifoggant, a compound having a nucleus of the formula:##STR12##
 2. A photothermographic element according to claim 1 whereinsaid antifoggant compound has the general formula: ##STR13## whereineach R is independently a member selected from the group consisting ofhydrogen, alkyl of up to 10 carbon atoms, aryl of up to 14 carbon atoms,heterocyclic groups of up to 14 ring atoms, halogen, carboxylic acid,ester, amide, hydroxy and any combination thereof.
 3. Aphotothermographic element according to claim 2 wherein each R ishydrogen.
 4. A photothermographic element according to claim 1 whereinsaid antifoggant compound is present in an amount from 1×10⁻³ to 1×10⁻¹moles per mole of silver halide.
 5. A photothermographic elementaccording to claim 4 wherein said reducible silver source is the silversalt of behenic acid.
 6. A photothermographic element according to claim5 wherein said reducing agent for silver ion is a member selected fromthe group consisting of a phenidone, hydroquinone, catechol and hinderedphenol having a nucleus of the general formula: ##STR14## wherein R⁶ isa member selected from the group consisting of hydrogen and an alkylgroup comprising up to 15 carbon atoms, and R⁷ and R⁸ are independentlymembers selected from the group consisting of hydrogen and alkyl groupsof up to 5 carbon atoms.
 7. A photothermographic element according toclaim 6 wherein said reducing agent is used in combination with a tonerwhich is a member selected from the group consisting of phthalazinone,phthalazine, phthalic acid and any combination thereof.
 8. Aphotothermographic element according to claim 4 which additionallycomprises, as a speed enhancing agent, a hydrobromic acid salt having acentral nucleus selected from the group consisting of: ##STR15## whereinQ comprises the atoms necessary to complete a 5, 6, or 7-memberedheterocyclic ring nucleus which is a member selected from the groupconsisting of a pyridine, pyrrolidone, and pyrrolidinone ring nucleus.9. A photothermographic element according to claim 8 wherein saidhydrobromic acid salt comprises a compound having a central nucleus of aformula selected from the group consisting of: ##STR16## wherein n is 0(zero) or has integral values of from 1 to 4, andeach R⁵ isindependently a member selected from the group consisting of hydrogen,alkyl of up to 10 carbon atoms, aryl of up to 14 carbon atoms,heterocyclic groups of up to 14 ring atoms, halogen, carboxylic acid,ester, amide, hydroxy and any combination thereof.
 10. Aphotothermographic element according to claim 9 wherein said hydrobromicacid salt is pyridinium hydrobromide perbromide.